1. Field of the Invention
The present invention relates to a solid-state laser device for use as a light emission device or a similar device and a solid-state laser amplifier provided therewith, and more particularly to a solid-state laser device of a reduced size and a solid-state laser amplifier provided therewith.
2. Description of the Related Art
A so-called semiconductor laser pumped solid-state laser device (LD pumped solid-state laser device) using a semiconductor laser (laser diode) as an excitation light source has such a desired characteristic for industrial use as a long service life, a high efficiency and high brightness. For the reason, the LD pumped solid-state laser device has been actively developed. Particularly, the LD pumped Nd:YAG laser has been developed so that its output is 10W level on the market or kW level on development stage.
By using a laser crystal having a high absorption peak in a pump wavelength range, an output of the solid-state laser device can be provided in a very small volume. Thus, attention has been paid to development on reduction of the size of a laser head size. As such a solid-state laser device, for example, so-called microchip-type solid-state laser device is currently available. FIG. 1 is a schematic diagram showing an existing microchip-type solid-state laser device. In the existing microchip-type solid-state laser device, one end face 102 of a laser crystal 101 in which a thickness thereof is about several hundreds xcexcm, generally about one or two times absorption length which is a reciprocal of the absorption coefficient, is treated with high reflection coating, while the other end face 103 is treated with partial reflection coating.
If laser beam impinges upon the end face 102 on which the high reflection coating is applied from inside of the laser crystal 101, the laser beam is fully reflected. However, excitation beam impinging on the end face 102 from outside of the laser crystal 101 is not reflected but passes through. If the laser beam impinges upon the end face 103 on which the partial reflection coating is applied from inside of the laser crystal 101, part of the laser beam is reflected, and the other part thereof is not reflected but passes through.
A focusing lens 104 and a collimate lens 105 are disposed in parallel to the end face 102 on a side of the end face 102 of the laser crystal 101. A semiconductor laser 106 is disposed at a position sandwiching the focusing lens 104 with the collimate lens 105. Excitation laser beam 107 is emitted from an emission portion 106a of the semiconductor laser 106 to the collimate lens 105.
In a existing microchip-type solid-state laser device having such a structure, the excitation laser beam 107, which is emitted from the emission portion 106a of the semiconductor laser 106 and has a divergence angle, is converted to parallel beam by the collimate lens 105. After converted to parallel beam, the excitation laser beam 107 is refracted by the focusing lens 104 so that its focal point is made on the end face 102. The laser crystal 101 is excited by the beam impinging upon the end face 102 and the beam is reflected between the end face 102 and end face 103 so as to generate resonator internal circulating beam 107a. After that, laser oscillation beam 109 is emitted from the end face 103.
Table 1 shows absorption coefficients and absorption lengths of typical solid-state laser crystals (Nd:YVO4 crystal, Nd:YAG crystal and Yb:YAG crystal) having an oscillation wavelength band of 1 xcexcm.
Nd:YVO4 having an excessively large absorption coefficient of Nd series has been often used as a laser crystal for the microchip-type solid-state laser device. Further, Yb:YAG having a quasi-three-level energy structure can be also formed to a microchip. By that microchip-type structure, it is possible to integrate a laser resonator comprising a solid-state laser crystal and a reflector sandwiching it with the solid-state laser crystal. As a result, a small solid-state laser device having a high resistance to mechanical vibration and thermal variation can be achieved.
For example, a solid-state laser device having a structure shown in FIG. 1 and employing Nd:YVO4 crystal as the laser crystal 101 has been described in xe2x80x9cOptics Letters, vol.16, p.1955, 1991xe2x80x9d. The thickness of the Nd:YVO4 crystal which is used as the laser crystal of the solid-state laser device described in the document is 500 xcexcm. By exciting the Nd:NVO4 with a semiconductor laser (wavelength: 809 nm) having 500 mW output, a laser output of 160 mW (wavelength: 1064 nm) can be obtained. At this time, in the excitation optical system, a single lens having a focal length of 4.5 mm, which functions as a collimate lens and a focusing lens at the same time, is used. The reflection factor of the partial reflector is 99% with respect to the laser oscillation wavelength.
However, in a conventional LD pumped solid-state laser device as described previously, as shown in FIG. 1, an optical element (lens, mirror, prism and such) having a limit in miniaturization of the excitation optical system for introducing the excitation laser beam 107 into the laser crystal 101 has been often used. Therefore, in the conventional microchip-type solid-state laser device, even if the laser crystal can be thinned, the entire laser head including an excitation optical system and a LD is not always of small size. In such an excitation optical system having a micro optical element, after each optical element is adjusted to an accurate position on the optical axis, it needs to be held strictly. Further, there is such a problem that a change of the semiconductor laser focusing position and a change of the laser output thereby is caused by thermal expansion of the case due to temperature change, mechanical vibration and such, because of two reasons that a propagation distance of the excitation beam is large and that the macro optical element is spatially separated from the laser crystal for not integral type.
Therefore, a solid-state laser device not having the excitation optical system provided with a lens and such has been proposed in xe2x80x9cOptics Letters, vol.17, p.1201, 1992xe2x80x9d. FIG. 2 is a schematic diagram showing a conventional solid-state laser device described in xe2x80x9cOptics Letters, vol.17, p.1201, 1992xe2x80x9d. In the conventional solid-state laser device, a partial reflector 112a is disposed on an end face of the laser crystal 111 and a full reflector 113 is disposed on the other end face. Further, a Q switch element 120 is connected to the partial reflector 112a and a partial reflector 112b is disposed on the other end face of the Q switch element 120. Further, an electrode 120a on which a source voltage is applied and an electrode to be grounded are provided for the Q switch element 120. Then, a semiconductor laser 116 is disposed on the end face in which the full reflector 113 of the laser crystal 111 is provided.
Because the solid-state laser device does not contain any excitation optical system like the collimate lens, miniaturization thereof can be achieved more easily than the above-mentioned conventional solid-state laser device. However, because the divergence angle of the excitation laser beam 117 is large, it is necessary to dispose the semiconductor laser 116 and the laser crystal 111 near each other. Therefore, it is difficult to connect plural semiconductor lasers for excitation so that there is an obstacle in enhancement of the output.
A solid-state laser device in which the excitation laser beam is transmitted via multi-mode fiber has been described in xe2x80x9cConference on Lasers and Electro-optics (CLEO), p.176, paper CWC6, 1995xe2x80x9d. FIG. 3 is a schematic diagram showing a conventional solid-state laser device described in xe2x80x9cConference on Lasers and Electro-optics (CLEO), p.176, paper CWC6, 1995xe2x80x9d. In this conventional solid-state laser device, multi-mode optical fiber 116a is disposed instead of the aforementioned semiconductor laser 116. Further, although not shown, an excitation laser beam impinges on the other end of the optical fiber 116a from a semiconductor laser.
In this case also, the Q switch element 120 is provided so that pulse operated microchip laser is achieved. However, a laser head including the semiconductor laser is not always of small size.
Further, a solid-state laser device for generating the fourth harmonics has been described in xe2x80x9cConference on Lasers and Electro-optics (CLEO), p.236, paper CWA6, 1996xe2x80x9d. FIG. 4 is a schematic diagram showing a conventional solid-state laser device described in xe2x80x9cConference on Lasers and Electro-optics (CLEO), p.236, paper CWA6, 1996xe2x80x9d. In this conventional solid-state laser device, a laser crystal 131 composed of Nd:YAG crystal is connected to an optical fiber 135 and a KTP crystal 132 is connected to the laser crystal 131 through a Cr:YAG crystal 133, which is the Q switch element. Further, a fourth harmonics generating element 134 is connected to the KTP crystal 132 and a window 136 is provided with a predetermined distance relative to the fourth harmonics generating element 134.
With a progress of development of micro optical devices (micro optics), there has been produced a prior art in which a lens array or a prism array is used as the excitation optical system (xe2x80x9cApplied Optics vol.35, p.1430, 1996xe2x80x9d). FIG. 5 is a schematic diagram showing a conventional solid-state laser device described in xe2x80x9cApplied Optics vol.35, p.1430, 1996xe2x80x9d. In this conventional solid-state laser device, on one end face of a rod type laser crystal 141, in which reflectors are disposed on both end faces, a focusing lens 144, a cylindrical lens 145, a rotation prism array 148, a cylindrical lens array 147 and a laser bar 146 composed of a plurality of elements are arranged in order from the laser crystal 141 side. Further, a Q switch 149 and an output mirror 150 are disposed on the same line as the focusing lens 144 and such on the other end face side of the laser crystal 141.
In this conventional solid-state laser device, a rotation prism of the rotation prism array 148 is rotated at each element of the semiconductor laser bar 146. The focal point of the excitation laser beam is focused on the end face of the laser crystal 141 made of Nd:YAG crystal by the focusing lens 144. Then, the laser crystal 141 is excited.
Further, an example of the excitation optical system using a micro step mirror has been described in xe2x80x9cOSA TOPS vol.10 Advanced Solid-State Lasers, 1997, p.390xe2x80x9d. FIG. 6 is a schematic diagram showing a solid-state laser device described in xe2x80x9cOSA TOPS vol.10, Advanced Solid-State Lasers, 1997, p.390xe2x80x9d. This conventional solid-state laser device is provided with a micro step mirror 151 comprising a pair of mirrors each having a mirror surface inclined against the direction of advancement of laser. Semiconductor laser bar beam 152 irradiated to the micro step mirror 151 is reflected by one mirror and projected to the other mirror. Further, beam projected to the other mirror is reflected by the same mirror. As a result, excitation beam group 153 rotated by 90 degree from the semiconductor laser bar beam 152 is emitted from the micro step mirror 151.
However, although the optical element like the lens is not necessary in these conventional solid-state laser devices, there is such a problem that optical loss is large so that operating stability is not enough.
It is an object of the present invention to provide a small solid-state laser device in which a high stability and a high output can be assured, and a solid-state laser amplifier provided with the same.
According to one aspect of the present invention, a solid-state laser device may comprise a substrate and a laser crystal formed on the substrate. The laser crystal oscillates laser beam from a second end face when an excitation beam is projected to a first end face. The solid-state laser device may further comprise an excitation beam emission element emitting the excitation beam and a waveguide path on the substrate, the excitation beam being transmitted through the waveguide path. The waveguide path has a first end portion on which the excitation beam emitted by the excitation beam emission element impinges and a second end portion which opposes the first end face of the laser crystal, the excitation beam being emitted from the second end portion.
According to another aspect of the present invention, a solid-state laser device may comprise a substrate and a laser crystal formed on the substrate. The laser crystal oscillates laser beam from a second end face when an excitation beam is projected to a first end face. The solid-state laser crystal may further comprise an excitation beam emission element emitting the excitation beam, a plurality of waveguide paths formed on the substrate, the excitation beam being transmitted through each of the waveguide paths, and a synthesizer formed on the substrate. Each of the waveguide paths has a first end portion on which the excitation beam emitted by the excitation beam emission element impinges and a second end portion from which the excitation beam is emitted. The synthesizer connects each of the second end portion of the waveguide path to one another and opposes the first end face of the laser crystal.
According to the present invention, the excitation beam emitted by the excitation emitting element may be transmitted through the waveguide path and emitted to the first end face of the laser crystal. Then, laser beam is oscillated from the second end face by the laser crystal. Thus, loss during transmission is small and such optical devices as a lens and prism are not necessary. Therefore, the size of the excitation optical system can be extremely reduced and therefore the size of the laser head can be reduced.
Further, because the excitation optical system and laser crystal can be integrated on the same substrate, it is possible to provide a high reliability solid-state laser beam source having a small output variation with respect to temperature changes and mechanical vibration.
Further, because various kinds of optical control elements (optical modulation, wavelength filter, optical switch and such) can be provided on the waveguide path, it is possible to obtain a smaller, higher performance laser beam source by integrating a high-performance optical element.
The first end face may be coated with a film not reflecting the excitation beam and fully reflecting the laser beam excited by the excitation beam, and the second end face may be coated with a film partially reflecting the laser beam.
The solid-state laser device of the present invention may further comprise a wavelength conversion element converting the wavelength of the laser beam oscillated from the second end face. The first end face may be coated with a film not reflecting the excitation beam and fully reflecting the laser beam excited by the excitation beam and the second end face may be coated with a film not reflecting at least a part of the laser beam. As a result, a higher output can be obtained.
Further, the thickness of the laser crystal is preferably one to two times a reciprocal of an absorption coefficient of the excitation beam.
Furthermore, the solid-state laser device of the present invention may comprise a focusing lens focusing the laser beam and a transmission path of single mode through which the laser beam focused by the focusing lens is transmitted. The single mode transmission path may comprise, for example, a single-mode fiber or a single-mode quartz waveguide path.
According to another aspect of the present invention, a solid-state laser amplifier may comprise a substrate and a laser crystal formed on the substrate. The laser crystal emits laser beam from a second end face when an excitation beam is projected to a first end face. The solid-state laser amplifier may further comprises a non-reflecting film coating the first and second end faces, an excitation beam emission element emitting the excitation beam and an waveguide path for excitation beam formed on the substrate, the excitation beam being transmitted through the waveguide path for excitation beam. The non-reflecting film does not reflect a signal beam within a gain bandwidth of the laser crystal. The waveguide path for excitation beam has a first end portion on which the excitation beam emitted by the excitation beam emission element impinges and a second end portion which opposes the first end face of the laser crystal, the excitation beam being emitted from the second end portion. The solid-state laser amplifier may further comprise a waveguide path for signal beam entry formed on the substrate, the signal beam within the gain bandwidth being transmitted through the waveguide path for signal beam entry, and a waveguide path for signal beam emission on the substrate, the laser beam oscillated from the second end face being transmitted through the waveguide path for signal beam emission. The waveguide path for signal beam entry has an end portion, the signal beam being emitted toward the first end face of the laser crystal from the end portion. The waveguide path for signal beam emission has an end portion on which the laser beam impinges.
According to another aspect of the present invention a solid-state laser amplifier may comprise a substrate and a laser crystal formed on the substrate. The laser crystal emits laser beam from a second end face when excitation beam is projected to a first end face. The solid-state laser amplifier may further comprise a full-reflection film coating the first end face, a non-reflection film coating the second end face, an excitation beam emission element emitting the excitation beam and an waveguide path for excitation beam formed on the substrate, the excitation beam being transmitted through the waveguide path for excitation beam. The full-reflection film does not reflect the excitation beam and fully reflects a signal beam within a gain bandwidth of the laser crystal. The non-reflection film does not reflect the signal beam within the gain bandwidth. The waveguide path for excitation beam has a first end portion on which the excitation beam emitted by the excitation beam emission element impinges and a second end portion which opposes the first end face of the laser crystal, the excitation beam being emitted from the second end portion. The solid-state laser amplifier may further comprise a waveguide path for signal beam entry on the substrate, the signal beam within the gain bandwidth being transmitted through the waveguide path for signal beam entry, and a waveguide path for signal beam emission formed on the substrate, the laser beam oscillated from the second end face being transmitted through the waveguide path for signal beam emission. The waveguide path for signal beam entry has an end portion, the signal beam being emitted toward the second end face of the laser crystal. The waveguide path for signal beam emission has an end portion on which the laser beam impinges.
According to the present invention, the signal beam which is transmitted through the waveguide path for signal beam entry and emitted to the end face of the laser crystal is amplified in the laser crystal and emitted into the waveguide path signal beam emission. In this case also, loss is small and the size of the amplifier can be reduced easily because a lens and such is not necessary.
According to the present invention, because an active element having an amplifying function and a passive element having optical control function can be integrated on the same substrate, a small size, high performance laser amplifier can be obtained in application for optical transmission, optical information processing and such.
Further, batch film fabrication process technology in semiconductor industry and self-align technology in optoelectronics industry may be applied to the present invention. By employing these technologies, production cost can be reduced.